In the oil and gas industry, wells are drilled deep into the earth's crust for the purpose of finding and retrieving petrochemicals. Operating companies, who own or manage such wells, as well as oilfield services companies, evaluate wells in a variety of ways, for example, by acquiring formation cores. These formation cores may be obtained using coring tools—tools which may be conveyed on a wireline suspended into the well and which drills into the side-wall of the borehole to obtain formation samples, also known as cores.
The assessment of formation characteristics acquired from formation cores is often crucial to the decision-making process concerning development plans for petroleum wells that are being evaluated as part of an exploration or production activity. Take, for example, a well that has been drilled and evaluated by well logging or the acquisition of formation cores. Depending on the results of the evaluation, the well could be drilled deeper, plugged and abandoned as non-productive or cased and tested. The evaluation may also be inconclusive and the determination made that additional evaluation, for example, further acquisition of side-wall cores of the formation, is required before a decision on the disposition of the well can be made. The results of the core analysis as interpreted from a well log may also help determine whether the well requires stimulation or special completion technologies, such as gas lift or sand control. The decisions made from well evaluations are very difficult, often made with imperfect information, have huge economic impact, and frequently have to be made very quickly. Mistakes, or even mere delay, can be extremely expensive.
There are several different types of tools for obtaining side cores. One approach is to manipulate a rotating hollow cylindrical coring bit into the side-wall of the borehole. As the rotating coring bit is forced into the sidewall, a small sample of the formation, known herein as the core, is collected in the interior of the coring bit. An example of a side-coring tool is the Mechanical Side-Coring Tool (MSCT™) of Schlumberger Technology Corporation. Side-wall core samples are acquired by the MSCT™ using rotary drilling whereby no percussion damage is caused by rotary drilling into the side-wall of the borehole. The Mechanical Side Coring Tool is operable to acquire up to twenty side-wall core samples during a single trip into the borehole. The rotary drilling of the side-wall core by the MSCT™ preserves the properties of the side-wall core samples thereby allowing accurate measurements of parameters such as relative permeability and secondary porosity.
Production company personnel at a well site or other personnel involved in planning a logging job may plan for a side-wall coring job that involves acquiring side-wall cores for particular depths of interest. A coring tool is then lowered to the depth of interest and coring operations are performed at these depths. Core samples are collected in the tool and the entire apparatus retrieved to the surface. Upon retrieving the coring tool, these personnel may discover, to their dismay, that a fewer number of cores were actually acquired during the job than what was planned for. An additional problem from the failure to acquire all planned side-wall cores is a difficulty in sorting out which side-wall core associates to a specific planned depth of interest. Furthermore, the lack of core analysis in current coring tools result in delay in testing and updating any reservoir model until such time the acquired side-wall cores are analyzed in the laboratory.
Oil and gas wells can be extremely deep. It is not uncommon for the wells to be as much as 30,000 feet in vertical depth. Often a depth of interest is located near the bottom of such deep wells. Consequently, the operation of retrieving a wireline and its attached tool-string to the surface can be a very time consuming and expensive operation. The same can be said for the redeployment of the wireline and tools into the well to acquire additional information, be it geophysical measurements from sensors or additional core samples.
One method of in-situ analysis of cores captured in inline coring operations is disclosed in U.S. Pat. Nos. 6,220,371, 6,003,620 and 5,984,023 to Sharma et al. In an inline coring operation, core samples are obtained by a coring bit operating at the end of a core barrel extending in the borehole from the surface to the bottom of the well. Core samples are brought up to the surface in an inner core barrel located inside an outer core barrel. In the analysis system of Sharma et al., core samples are moved in the inner core barrel to the surface and the measurements of the core samples are taken as they move past an array of sensors. Coring at the end wall of the borehole and in the direction of the borehole is generally referred to as “conventional” coring. Multiple core acquisition is generally unavailable with conventional coring and would undesirably increase the cost and complexity of acquiring and analyzing of the multiple cores.
From the foregoing it will be apparent to those skilled in the art that there is a need for an improved method to monitor the acquisition of side-wall cores by a coring tool. Furthermore, knowledge that side-wall cores have been acquired at each specified depth of interest in the well is desirable. It will also be apparent to those skilled in the art that there is a need for an improved method to analyze the side-wall core while the core is still in the coring tool and the coring tool is still in the borehole. Furthermore, providing timely core analysis results, in near real-time, whereby the analysis results can be used to test and update any reservoir model based on the continuous log available at the wellsite. There is a further need to make core analysis results available in near real-time to decision-makers thereby permitting decisions as to which course of action to take with respect to the coring operation.